The present invention relates generally to flame-retardant advanced epoxy resins prepared from a bisphenol having phosphorus group. The present invention also relates to cured epoxy resins resulting from the advanced epoxy resins, which have excellent flame-retardancy and mechanical properties
Epoxy resins have the excellent characteristics of moisture, solvent and chemical resistance, toughness, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. The versatility in formulation also make epoxy resins widely applicable industrially for surface coatings, adhesive, painting materials, potting, composites, laminates, encapsulants for semiconductors, and insulating materials for electric devices, etc. O-Cresol formaldehyde novolac epoxy (CNE) is the resin typically employed in the encapsulation of microelectronic devices. Several approaches for modification of epoxy backbone for enhancing the thermal properties of epoxy resins have been reported. Aromatic bromine compounds in conjunction with antimony oxide are widely used as a flame-retardant for epoxy resins. Tetrabromobisphenol A is a typical example of the aromatic bromine compounds used as a flame-retardant for epoxy resins. An excess amount of epoxy resin is reacted with tetrabromobisphenol A to prepare an advanced epoxy resin having two terminal epoxide groups, as shown in the following formula: 
A flame-retardant advanced epoxy resin, wherein EP is a bi-radical group of the backbone of the epoxy resin, and m is an integer of 1-10. The advanced epoxy resin can be used in preparing a flame-retardant printed circuit board (FR-4) by impregnating glass fibers with the advanced epoxy resin and heating the resulting composite to cure the advanced epoxy resin. Furthermore, the advanced epoxy resin can be employed to encapsulate microelectronic devices, in which the advanced epoxy resin is cured at a high temperature with a curing agent, so that an encapsulant having a flame-retardant property is formed. Typical examples can be found in U.S. Pat. No. 3,040,495 (1961); U.S. Pat. No. 3,058,946 (1962); U.S. Pat. No. 3,294,742 (1966); U.S. Pat. No. 3,929,908 (1975); U.S. Pat. No. 3,956,403 (1976); U.S. Pat. No. 3,974,235 (1976); U.S. Pat. No. 3,989,531 (1976); U.S. Pat. No. 4,058,507 (1997); U.S. Pat. No. 4,104,257 (1978); U.S. Pat. No. 4,170,711 (1979); and U.S. Pat. No. 4,647,648(1987).
Although the tetrabromobisphenol A-containing advanced epoxy resin shows flame-retardant property, major problems encountered with this system are concerned with the generation of toxic and corrosive fumes during combustion such as dioxin and benzofuran.
The flame-retardant having a small molecular weight tends to degrade the mechanical properties of the epoxy resins, and migrate/vaporize from the epoxy resins such that the flame retardancy thereof diminishes.
The trend of electronics equipment is being miniaturized and becoming thinner, at the same time the scale of integration of large scale integrated circuits (LSICs) is continuing upward, forcing the design toward larger chips, finer patterns, and higher pin counts that are more susceptible to a high-temperature failure. The prevailing surface mount technology (SMT) also causes the devices to be subjected to a high temperature. Therefore, the development of a high-temperature reliable, flame-retardant and environmentally friendly epoxy resin for printed circuit board and encapsulant are essential for semiconductor industry.
It is an object of this invention to provide flame-retardant advanced epoxy resins and cured epoxy resins with good thermal stability, superior heat resistance, and environment friendly, which are suitable for use in making printed circuit boards and in semiconductor encapsulation applications.
It is another object of this invention to provide a method for improving flame-retardant properties of epoxy resins.
In order to accomplish the aforesaid objects, a flame-retardant advanced epoxy resin and a cured epoxy resin disclosed in the present invention have the following formula (I): 
wherein:
m is greater than 0 and less than 10;
X=A or B, wherein: 
when X=A, the formula (I) represents the advanced epoxy resin, X=B, the formula (I) represent the cured epoxy resin; 
wherein R1 is hydrogen or C1-4 hydrocarbon group;
wherein 
or a phenol-aldehyde novolac epoxy resin backbone, and when Ep is the phenol-aldehyde novolac epoxy resin backbone, the flame-retardant advanced epoxy resin and the cured epoxy resin represented by the formula (I) is prepared by reacting a dihydric phenol or naphthol having the following formula (III): 
wherein R is the same as defined above, with a phenol-aldehyde novolac epoxy resin having the following formula (IIxe2x80x2): 
wherein R1 is the same as defined above, g is an integer of 1-6, and R4 and R5 independently are hydrogen or xe2x80x94CH3.
The flame-retardant advanced epoxy resin and cured epoxy resin of the present invention contain a rigid phosphorus group, which provide not only the better flame-retardant effect and thermal stability than those prepared with a conventional aromatic bromine group, but also generate much less fumes in the combustion test.
The present invention provides an advanced epoxy resin and a cured epoxy resin containing a flame-retardant phosphorus group, which may be represented by the following formula (I), 
wherein m, X, Ep and R are defined as above.
A suitable method for preparing the flame-retardant advanced epoxy resin and cured epoxy resin represented by the formula (I) comprises reacting an epoxy resin having the following formula (II) and a phosphorus-containing dihydric phenol or naphathol having the following formula (III) in a molten state or in a common solvent and in the presence of a catalyst: 
wherein Ep and R are the same as defined above.
Preferably, Ep in the formula (I) is 
wherein Q is xe2x80x94C(CH3)2xe2x80x94, or Ep is a phenol-aldehyde novolac epoxy resin backbone having the following formula: 
wherein R1 is hydrogen and R4 is xe2x80x94CH3.
Preferably, R in the formula (I) is 
wherein R1 is defined as above.
In the method for preparing the flame-retardant advanced epoxy resin and cured epoxy resin compound (I), the advanced epoxy resin (Ixe2x80x2), i.e. X in the formula (I) is 
is prepared when an excess amount of epoxy resin (II) is used to react with phosphorus-containing dihydric phenol or naphthol (III) as shown in the following reaction: 
wherein R, Ep, and m are the same as defined above. Preferably, this reaction is carried out at 100xc2x0 C.-200xc2x0 C., more preferably at 150xc2x0 C.-180xc2x0 C., and in the presence of a catalyst selected from the group consisting of 2-phenylimidazole, 2-methylimidazole, triphenylphosphine, a quarternary phosphoium compound and a quarternary ammonium compound. Examples of the quarternary phosphoium compound include ethyltriphenyl phosphonium acetate and ethyltriphenyl phosphonium halides. Examples of the quarternary ammonium compound are benzyltrimethyl ammonium chloride, benzyltriethyl ammonium chloride and tetrabutyl ammonium chloride. Preferably, the equivalent ratio of the epoxy resin (II) to the hydroxyl group in the bisphenol compound (III) ranges from 1.5:1 to 5:1; and more preferably 2:1 to 3:1.
In the method for preparing the flame-retardant cured epoxy resin, i.e. X in the formula (I) is B, the phosphorus-containing dihydric phenol or naphthol (III) was used as a curing agent of the epoxy resin (II). The dihydric phenol or naphthol (III) can be used alone or together with another curing agent such as phenol-formaldehyde novolac, dicyandiamide, methylenedianiline, diaminodiphenyl sulfone, phthalic anhydride and hexahydrophthalic anhydride. A suitable amount of the curing agent for curing the epoxy resin (II) is the equivalent ratio of the epoxide group in the epoxy resin (II) and the functional groups in the curing agent ranging from 1:1 to 1.2:1.
It is apparent that the present invention also provides a method for improving flame-retardant properties of the epoxy resin (II), which comprises reacting said epoxy resin with the phosphorus-containing dihydric phenol or naphthol (III), in which the epoxide group of said epoxy resin is reacted with the hydroxyl group of said phosphorus-containing containing dihydric phenol or naphthol so that an open ring reaction of said epoxide group occurs.
A suitable epoxy resin for use in the present invention can be any known epoxy resin, for examples those having two epoxide groups such as bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin and bisphenol epoxy resin, and those having more than two epoxide groups such as phenol formaldehyde novolac epoxy and cresol formaldehyde novolac epoxy (CNE).
The phosphorous-containing containing dihydric phenol or naphthol (III) used in the present invention can be synthesized according to the following reaction (IV): 
wherein R1 and R are the same as defined above. The reaction (IV) is preferably carried out at a temperature range of 80xc2x0 C.xcx9c120xc2x0 C. in an appropriate solvent such as toluene.